Pore size classes (IUPAC)
| Class | Pore width |
|---|---|
| Micropores | < 2 nm (ultramicropores < ~0.7 nm; supermicropores ~0.7–2 nm) |
| Mesopores | 2–50 nm |
| Macropores | > 50 nm |
| Nanopores (umbrella term) | up to ~100 nm |
The eight isotherm types
| Type | Shape | Typical materials | What it tells you |
|---|---|---|---|
| I(a) | Steep uptake at very low p/p₀, then a plateau | Microporous solids with mainly narrow micropores (< ~1 nm), e.g. many zeolites | Micropore filling; the plateau gives the micropore volume |
| I(b) | Uptake extends to somewhat higher p/p₀ before the plateau | Wider micropores and narrow mesopores (< ~2.5 nm), e.g. some activated carbons | Broader micropore distribution |
| II | Reversible; knee (point B) then steadily rising multilayer | Non-porous or macroporous powders | Unrestricted monolayer–multilayer adsorption; ideal for BET |
| III | Convex to the p/p₀ axis, no point B | Weakly interacting systems, e.g. water on hydrophobic non-porous surfaces | Adsorbate–adsorbate interactions dominate; BET not applicable |
| IV(a) | Like Type II at low p/p₀, then capillary condensation with a hysteresis loop and usually a plateau | Mesoporous oxides, MCM-41/SBA-15 with pores wider than ~4 nm (N₂, 77 K) | Mesopores; analyse with BJH or DFT |
| IV(b) | As IV(a) but fully reversible | Cylindrical mesopores narrower than ~4 nm, or conical/closed-end mesopores | Narrow mesopores |
| V | Like Type III at low p/p₀, then pore filling, with hysteresis | Water on hydrophobic microporous or mesoporous adsorbents | Weak interaction plus pore filling |
| VI | Stepwise | Layer-by-layer adsorption on highly uniform non-porous surfaces, e.g. Ar or Kr on graphitised carbon black at low temperature | Surface homogeneity |
How to classify your isotherm
- Plot it on a linear p/p₀ axis (and on a log axis for the low-pressure region if micropores are suspected).
- Look at the low-pressure region. A near-vertical rise below p/p₀ ≈ 0.01 means micropores (Type I). A rounded knee means a monolayer forms first (Types II, IV). No knee at all means weak interaction (III, V).
- Look for a loop. Hysteresis between p/p₀ ≈ 0.4 and 1 means capillary condensation in mesopores. Classify it with the H1–H5 scheme.
- Look at the top end. A plateau means the pores are filled and there is little external or macropore surface. Continued rise near p/p₀ = 1 means macropores or interparticle voids.
- Accept composites. Real materials often combine features, for example Type I + IV for hierarchical zeolites. Describe what you see rather than forcing one label.
Wording it correctly
- “Type IV(a) isotherm with an H1 hysteresis loop” — ordered mesoporous silica such as SBA-15.
- “Type II isotherm with an H3 loop” — aggregates of plate-like particles (e.g. clays). Often written as “Type IV with H3”, but an H3 adsorption branch has no plateau.
- “Composite Type I/IV(a) isotherm with an H4 loop” — micro-mesoporous carbons or zeolites with intracrystalline mesopores.
- Cite the source of the classification: M. Thommes et al., Pure Appl. Chem. 2015, 87, 1051–1069.
From isotherm type to analysis method
| Isotherm | Surface area | Pore analysis |
|---|---|---|
| I(a), I(b) | BET with Rouquerol criteria (report as BET area) | t-plot / αs, DR, NLDFT/QSDFT; Ar at 87 K or CO₂ at 273 K for ultramicropores |
| II | BET (standard range) | External surface only; MIP for macropores |
| IV(a), IV(b) | BET (standard range) | BJH (correct branch) or NLDFT |
| III, V | BET not applicable | Report uptake; consider a different adsorbate |
Related guides
- Hysteresis loops H1–H5
- BET surface area calculation
- BJH pore size distribution
- Nitrogen vs. argon adsorption
Frequently asked questions
What is a Type IV isotherm?
A Type IV isotherm is given by mesoporous adsorbents. Monolayer–multilayer adsorption is followed by capillary condensation in the mesopores, usually ending in a plateau. Type IV(a) shows a hysteresis loop (for N₂ at 77 K, pores wider than about 4 nm); Type IV(b) is reversible (narrower or closed-end mesopores).
Is "Type IV isotherm with H3 hysteresis" correct?
It is widely used, but in the IUPAC 2015 description an H3 loop has an adsorption branch resembling Type II, with no plateau at high p/p₀. It is typical of plate-like particle aggregates or incompletely filled macropore networks. "Type II isotherm with an H3 loop" is often the more accurate wording.
What are the IUPAC pore size classes?
Micropores are narrower than 2 nm, mesopores 2–50 nm and macropores wider than 50 nm. Micropores are subdivided into ultramicropores (< ~0.7 nm) and supermicropores (~0.7–2 nm). "Nanopores" covers all three up to about 100 nm.
Which isotherm types allow a valid BET area?
BET is most reliable for Type II and Type IV(a)/(b). For Type I it can be applied in a restricted low-pressure range selected with the Rouquerol criteria, and gives a characteristic "BET area" rather than a true surface area. It is not applicable to Types III and V.